In the playground the create scene function is wrapped by the code running the playground and so the easiest method is just to create a new scene within the function and add a camera, light and meshes to it. Also the playground has its own engine run render loop which needs to be stopped before running your own version, the trick is to use a setTimeout. Putting these together playground code can look like this example

Overlay Multiple Scenes

Suppose you want a first person shooter type project with a 3D foreground representing the user and then a background scene representing what the user is tracking. In this case you want the meshes in both the foreground and background visible. To do this you want the foreground meshes drawn over the background, that is you do not want the render canvas cleared when rendering the foreground. In this case set 'autoClear' to false